Since the 1996 ban, many of the leading researchers of the 1980s and 1990s have moved on to other specialties, and some said they've even discouraged students from specializing in gun violence research because the work doesn't pay. The ban also helped make gun-related questions controversial even for studies not funded by the government, and it will take years to restore available data to what it once was.

"Good research was being done by Art [Kellermann] and by us and by others on what the risk factors were for firearm violence, and how it might be prevented and so forth," said Dr. Garen Wintemute, an emergency room physician and leader on gun violence research at the University of California at Davis. "I won't say it halted, but it decreased substantially in scale."

The Ban

The CDC conducted gun violence research in the 1980s and 1990s, but it abruptly ended in 1996 when the National Rifle Association lobbied Congress to cut the CDC's budget the exact amount it had allocated to gun violence research.

"It's worth pointing out that the language never specifically forbade the CDC from conducting the research," Wintemute said.

The 1997 appropriations bill stated, "None of the funds made available for injury prevention and control at the Centers for Disease Control and Prevention may be used to advocate or promote gun control." Congress also threatened more funding cuts if the gun research continued.

"The message was really clear," Wintemute said.

In 2003, the 1997 bill language was updated to include the words "in whole or in part," which expanded the ban. Then, in 2012, the appropriations bill expanded the restriction to all Health and Human Services agencies.

Then, came the Sandy Hook Elementary School shooting on Dec. 14, 2012, when Adam Lanza opened fire and killed 20 students and six adults. A month later, Obama unveiled a sweeping plan to curb gun violence, but he still needed Congress to approve the research dollars.

Researchers Moved On

When the research was banned, Dr. Arther Kellermann had been working on CDC-funded research on whether home gun possession had potentially protective or hazardous effects on health. Nonprofit organizations helped fund the remainder of the project, but as the years went on, fewer and fewer nonprofits stepped up.

"I did not realize when I wrote an editorial in 1997 that the prohibition would continue for 17 years," Kellermann said, referring to his American Journal of Public Health piece about the NRA's response to gun research and its parallels to the tobacco industry's response to research linking cigarettes to cancer.

Like many gun violence researchers, Kellermann tried to stay in his field, but after a decade of waiting for the ban to lift, he had to find a different specialty. He now works on policy analysis at the Rand Corp.

Kellermann said he's watched as bumper stickers and slogans replaced statistics and facts over the past two decades. In the last week, a YouTube video of a 15-year-old Maryland girl's pro-gun speech attracted more than 2.3 million views. She cited statistics, but it's not clear where they came from.

"The shutting down or dramatically reducing the flow of new and relevant information in an era when our society often thinks from one tweet to the next ... means even credible work that's been done has basically been forgotten," he said.

No New Gun Research Students

Wintemute said even though existing gun data included many conditions that haven't changed, opponents can easily disregard it as being old and therefore irrelevant. He added that repeating this research requires real time research, which will take years to conduct and require a labor force that no longer exists.

As a result of researchers leaving his field and students steering clear of it because of funding concerns, Wintemute said the labor force available to dive back into gun violence research is virtually nonexistent. He estimated that only 12 or 15 people in the whole country are still researching gun violence because only a handful of nonprofits are willing to fund it.

"I don't think there's anyone at the CDC who has done significant work in this area in a decade," he said.

David Hemenway, a professor at the Harvard School of Public Health who specializes in injury research, said he cannot "in good conscience" tell his students to pursue careers in gun violence research.

"Don't write your dissertation about guns because it's not going to get any money," said Hemenway, who is considered one of the top gun violence researchers in the country. "All this knowledge is not going to be worth it to them, which is sad."

Wintemute said it's become a tough field to enter because researchers worry about death threats, adding that he himself was threatened by gun manufacturer Bruce Jennings, who founded B.L. Jennings Firearms and Bryco Arms in California. It has since gone bankrupt.

"The work is very controversial," he said. "There are some very angry people out there."

More Questions Than Answers

The act of inserting a single gun question into a survey has become increasingly difficult for Hemenway over the years, he said. For instance, when he asked to include whether families owned guns in a survey that had questions about how many alcoholic beverages people drank and how many cigarettes they smoked, he was turned down.

"If you want to know the simplest things, we don't know," he said. "We're not sure even what percent of houses have guns."

Some studies claim 33 percent have guns, while others estimate up to 59 percent, he said.

"It's not a crucial thing, but you'd think we would know that," Hemenway said. "We really have no idea how many guns there are."

When he was arguing to get the National Violent Death Reporting System started a few years ago, he used to give people a "pop quiz." He would ask, "Where are youths most likely to obtain the weapons they use in acts of violence?" and "When a firearm is used in a homicide or suicide, what is the typical length of time between the purchase of that firearm and the occurrence of the violent act?"

The answer was always the same: We don't know.

Still, many things have changed for the better since the Newtown, Conn., shooting, he said.

"First, the CDC can say the word 'guns,' and we have a president who is willing to talk about guns," Hemenway said, adding that a recent Quinnipiac University poll found that 91 percent of Americans wanted universal background checks for gun purchasers.

Nonpartisan groups like the United Physicians of Newtown formed to draw attention to guns as a public health issue. It started with few doctors looking for their children in a neighborhood firehouse right after the Sandy Hook shooting, and it's since swelled to 110 doctors – ranging from emergency room physicians to podiatrists.

"Whether it be any other disease, you can basically find the factor that goes into it ... so you can target and treat it," said Dr. James Bruno, a urologist whose daughter was in the school during the shooting. "If this is going to be addressed in a nonpartisan, constructive way, we want to look at the cause of these things."

Still, it will take time.

The NRA has contributed more than $21 million toward political candidates since 1990, and another $29 million on lobbying since 1998, according to the Center for Responsive Politics. The NRA did not respond to ABCNews.com's request for comment.

"When we care enough about children to do something, are we going to let a single lobby overrun what the vast majority of people want?" Hemenway asked.